The present invention relates to a silver halide photographic emulsion which is spectral-sensitized with at least two sensitizing dyes exhibiting supersensitization. More particularly, it relates to a silver halide photographic emulsion having increased spectral sensitivity, particularly in the green-sensitive region.
It is well known in the art that spectral sensitization (i.e., a technique to extend the light-sensitive wavelength region of a silver halide photographic emulsion to longer wavelength regions by adding thereto certain cyanine dye) is applied in the production of photographic light-sensitive elements. It is also known that the sensitivity is enhanced by spectral sensitization, i.e., spectral sensitivity is affected by the chemical structure of sensitizing dyes and various properties of an emulsion, e.g., the halogen composition, crystal habit and crystal system of silver halide, a silver ion concentration, and a hydrogen ion concentration. Furthermore, the spectral sensitivity is affected by photographic additives, such as a stabilizer, an antifoggant, an auxiliary coating agent, a precipitating agent, a color coupler, and a film hardener, which are present in the emulsion.
In general, one sensitizing dye is used to increase the spectral sensitivity of light-sensitive elements. When two or more sensitizing dyes are used in combination with each other, the spectral sensitivity is, in many cases, lower than those obtained using such sensitizing dyes singly. In specific cases, however, the spectral sensitivity can be greatly increased by using certain sensitizing dyes in combination with one or more sensitizing dyes. This phenomenon is known in the art as "supersensitization". Sensitizing dyes which can be used in combination with each other to obtain supersensitization have significant selectivity; that is, an apparently small distinction in the chemical structure exerts very large adverse influences on the supersensitization. Thus, combinations of sensitizing dyes which exhibit the supersenitization action could not be expected by merely observing the chemical structures thereof.
Furthermore, the sensitization of certain emulsions can be changed by changing the state of the emulsion. For example, the sensitization can be enhanced by increasing the silver ion concentration, decreasing the hydrogen ion concentration, or by performing both procedures in combination with each other. Therefore, the sensitization can be enhanced by soaking a film on which a spectral-sensitized emulsion has been coated in water or an aqueous solution of ammonia.
The foregoing method to change the sensitivity of a spectral-sensitized emulsion by increasing the silver ion concentration, decreasing the hydrogen ion concentration or by performing both procedures in combination with each other is usually called "hypersensitization". Emulsions subjected to the hypersensitization generally have poor storage stability.
Sensitizing dyes for use in the supersensitization of silver halide photographic emulsions need not undergo undesirable interaction with color couplers and other photographic additives and furthermore to have stable photographic characteristics during the storage of light-sensitive elements. It is also necessary that these sensitizing dyes leave no residual color in the light-sensitive element after processing. This is particularly necessary in rapid processing wherein the processing is performed in short periods of time (usually, several seconds to several ten seconds). Furthermore, these sensitizing dyes must reduce the fog density.
Combinations of sensitizing dyes which permit the supersensitization, in particular, of the green-sensitive region are known, as described in U.S. Pat. Nos. 3,580,724, 3,729,319 and 3,397,060, etc. These known combinations, however, fail to provide light-sensitive elements which have high green-sensitivity, permit a reduction in fog density and the residual color after processing, and which have good storage stability.